1. Field of the Invention
The invention relates to a radar device of the phase-monopulse type that detects objects.
2. Description of the Related Art
Radar has conventionally been installed in vehicles to detect distance, relative speed and direction of obstacles (such as in Japanese Patent No. 3433417 or Japanese Patent Application Publication No. 2001-166029 (JP-A-2001-166029)).
Japanese Patent No. 3433417 discloses a phase monopulse radar device capable of detecting objects in the vertical direction by arranging array antennas, in which element antennas that receive radio waves reflected by an object are arranged in the longitudinal direction, horizontally while shifting in the vertical direction. According to this device, since a vehicle can distinguish a road or a sign on a road, erroneous detection of an object can be prevented. In addition, JP-A-2001-166029 discloses digital beamforming (DBF) radar.
An explanation of a method for calculating the arrival direction of radio waves using a phase monopulse system is explained with reference to FIGS. 4A and 4B. FIG. 4A indicates the relationship between the arrival direction of radio waves and the difference in path length of the radio waves in the case of receiving radio waves with two reception antennas. Phases of the reception signals of the two reception antennas are different form each other due to the path length difference of the radio waves. FIG. 4B indicates the relationship between radio wave arrival angle (horizontal axis, [deg]) and phase difference (vertical axis, [deg]) in the case of installing reception antennas horizontally and using a value of λ/2 for the antenna interval d. Once the phase angle Δφ has been determined, the arrival direction of the radio waves can be calculated based on the formula θ=arcsin (λ·Δφ/(2·π·d)).
However, since detection of the phase difference of radio waves is carried out by detecting periodic shifts in the waveform of the radio waves, phase difference can only be detected within the range of −π to π [rad] due to this periodicity. Even if there is a phase difference that exceeds this range, it ends up being observed as a phase difference between −π to π[rad] (and this state is referred to as the occurrence of phase aliasing). Thus, it was conventionally considered necessary to make the antenna interval smaller than the half-wavelength of the wavelength of a carrier wave in order to be able to uniquely specify the arrival direction of radio waves (see FIG. 4B). However, in order to prevent exacerbation of characteristics attributable to physical dimensions of the antenna element and inter-antenna coupling, it is not possible to make the antenna interval as small as possible, but rather the antenna interval is frequently made to be half of the wavelength λ.
In the case of detecting the arrival direction of radio waves by a phase monopulse system, the azimuth angle at which radio waves arrive can be detected in the case of arranging two reception antennas in a row horizontally. Similarly, the elevation angle at which radio waves arrive can be detected in the case of arranging two reception antennas in a row vertically. Thus, both the azimuth angle and elevation angle at which radio waves arrive can be detected by arranging three or more reception antennas two-dimensionally (namely, such that all of the reception antennas are not arranged in a straight line).
However, in the case of detecting objects around a vehicle, since the elevation angle range in which detection targets are present is limited, the antenna directionality pattern thereof is preferably wider in the direction of azimuth angle and narrower in the direction of elevation angle. Since array antennas are typically long in the vertical direction and short in the horizontal direction, an antenna arrangement that realizes an antenna directionality pattern as described above has a low degree of freedom. For example, in the case of arranging two rectangular antennas along the lengthwise direction of the antennas, the interval between the phase centers of the antennas becomes excessively wide, and the angle range over which the arrival direction of radio waves can be accurately detected using a phase monopulse system decreases. In addition, there is also the disadvantage of the dimensions in the vertical direction of the antenna housing becoming excessively large. Thus, in the case of detecting the arrival direction of radio waves with a phase monopulse system using antennas, it is essential to arrange the antennas in the direction perpendicular to the lengthwise direction of the antennas. For this reason, it has been difficult to detect both the azimuth angle and elevation angle at which radio waves arrive with a phase monopulse method using a plurality of antennas.